I was climbing with a much more experienced partner recently and I cloved into the anchor. He said that I should always cinch the knot down right away or it could slip.

I did so to appease the leaders ego but it sounds like nonsense to me. Logic and experience tell me that obviously a loose clove will slip a foot or so as the knot tightens and then slip less and less with greater and greater force on an exponential curve until the rope breaks. If that weren't true people wouldn't aid solo on it, right?

I see it repeated here...
http://climbing.about.com/...tch-For-Climbing.htm